Bit Manipulation in C

Transcription

1 Bit Manipulation in C Bit Manipulation in C C provides six bitwise operators for bit manipulation. These operators act on integral operands (char, short, int and long) represented as a string of binary digits. Bitwise Operators logical operators: (unary) bitwise complement: ~ bitwise and: & &= bitwise or: = bitwise exclusive or: ^ ^= shift operators: left shift: << <<= right shift: >> >>= ICT106_Prac_Week_6 1 ICT106_Prac_Week_6 2 x y ~x x & y x y x ^ y Examples of application of bitwise operators on 8 bit representations of integers x and y int x, y, result; x = 15; /* */ y = -10; /* */ result = x & y; /* result now has the value decimal 6, i.e., */ ICT106_Prac_Week_6 3 ICT106_Prac_Week_6 4 Expression Representation Value x y result = x & y; result = x y; result = x ^ y; result = ~(x ^ y); result = ~x; result = x << 2; result = x >> 2; The bitwise AND operator & is often used to mask off some set of bits, e.g., x = x & 017; /* N.B. 017 is an octal value */ sets to zero all but the low-order 4 bits of x. The bitwise OR operator is often used to turn bits on, e.g., x = x y; sets to one in x the bits that are set to one in y. ICT106_Prac_Week_6 5 ICT106_Prac_Week_6 6 1

2 The bitwise exclusive OR operator ^ sets a one in each bit position where its operands have different bits, and zero where they are the same. The bitwise complement operator ~, also called the one s complement operator, inverts the bit string it is applied to. The 0s become 1s and the 1s become 0s (Toggle) The shift operators << and >> perform left and right shifts of their left operand by the number of bit positions given by the right operand, which must be positive. Structures in C ICT106_Prac_Week_6 7 ICT106_Prac_Week_6 8 Struct-ure Structs are like records and can be used to group data elements of different types, i.e., a struct is (usually) a collection of mixed type items. Two ways to define a structure: struct struct_name {variables typedef struct {variables Type_name; The typedef keyword allows to create name for a type, e.g. typedef unsigned char BYTE; BYTE one, bytes[5], *pr; ICT106_Prac_Week_6 9 ICT106_Prac_Week_6 10 char surname[4]; char firstinit; typedef struct { char surname[4]; char firstinit; EMPLOYEE; To Declare struct variables This defines a new data type called 'Employee'. This definition is for the use of C compiler, it does not allocate any space for the component variables. This definition can be used to declare variables as follows: Struct Employee manager; struct Employee worker1; EMPLOYEE manager; EMPLOYEE worker1; ICT106_Prac_Week_6 11 ICT106_Prac_Week_6 12 2

3 Accessing elements in a struct Initialising a struct is similar to that used for arrays: struct employee worker2 = { John, B, 32, Memory allocation for a structure J o h n B Char[0] surname Char[3] Initial Age Wage ICT106_Prac_Week_6 13 The elements (members) of a struct are accessed using the dot notation: worker1.firstinit = 'J'; worker1.age = 22; worker1.wage = ; manager.wage = 2* worker1.wage; ICT106_Prac_Week_6 14 Bit Field Another method of manipulating bits is to use a bit field; A bit field is constructed with a structure declaration that labels each field and determines its width; For example, to declare a Boolean data type #define TRUE 1 #define FALSE 0 struct BOOL { unsigned int bit : 1; ICT106_Prac_Week_6 15 ICT106_Prac_Week_6 16 Flag Register This definition creates a new data type BOOL that contains one 1-bit field. struct BOOL flag; flag.bit = TRUE; flag.bit = FALSE; struct FLAG { unsigned int overflow : 1; unsigned int sign : 1; unsigned int zero : 1; unsigned int aux_carry : 1; unsigned int parity : 1; unsigned int carry : 1; unsigned int direc : 1; unsigned int interrupt : 1; ICT106_Prac_Week_6 17 ICT106_Prac_Week_6 18 3

4 Nested Structures A struct can contain or nest another struct; Nested structures are often used to construct complex data record; For example, we can create a structure called names to represent an employee s name struct names { char first[4]; char surname[4]; struct names name; // nested struct int age; ICT106_Prac_Week_6 19 ICT106_Prac_Week_6 20 Pointers to Structures Again, the elements of a struct can be accessed using the dot notation: struct Employee worker1; strcpy((worker1.name).last, Bob ); strcpy((worker1.name).surname, John ); worker1.age = 43; The construction is interpreted from left to right Using pointers is easier to manipulate structures than the arrays; To overcome the problem of using structures as function arguments in some older C implementations; Enable to create dynamic size arrays ICT106_Prac_Week_6 21 ICT106_Prac_Week_6 22 /* employee record: demonstrating use of pointer to a structure */ #include <stdio.h> #define LEN 11 // define a constant LEN struct names { char last[len]; char surname[len] struct names name; // nested structure ICT106_Prac_Week_6 23 int main() { struct Employee workers[2] = { { { Ewen, Villard, 23, , { { Rodney, Swillbelly, 41, ICT106_Prac_Week_6 24 4

5 struct Employee *him; // a pointer to a structure him = &workers[0]; // assign address to pointer printf("workers[0].name.surname is %s\n", workers[0].name.surname); printf( workers[0].age is %d\n, workers[0].age); printf( (*him).age is %d\n, (*him).age); printf( him->age is %d\n, him->age); him++; // point to the next structure printf("him->name.surname is %s\nhim->wage is %.2f\n", him->name.surname, him->wage); return 0; workers[0].name.surname is Villard workers[0].age is 23 (*him).age is 23 him->age is 23 him->name.surname is Swillbelly him->wage is ICT106_Prac_Week_6 25 ICT106_Prac_Week_6 26 Linked List Linked lists contain a set of (mostly) similar data structures; An item can be added to or removed from a linked list and that the length of linked lists can be dynamically changed; The length of arrays is always fixed and it must be defined before use; Construction of an employee linked list is shown below: strut Employee *next; // pointer to the next element strut names name; ICT106_Prac_Week_6 27 ICT106_Prac_Week_6 28 int main() { struct Employee workers[2] = { { NULL, // pointer to nowhere {"Ewen", "Villard", 23, , { NULL, // pointer to nowhere {"Rodney", "Swillbelly", 41, ICT106_Prac_Week_6 29 struct Employee *him; // a pointer to a structure him = &workers[0]; // assign address to pointer him->next = him+1; // link to the next record printf("(*(him->next)).name.surname is %s\n", (*(him->next)).name.surname); him++; // point to the next structure printf("him->name.surname is %s\n, him->name.surname); ICT106_Prac_Week_6 30 5

6 (*(him->next)).name.surname is Swillbelly him->name.surname is Swillbelly struct Employee * age wage struct Employee * age wage NULL ICT106_Prac_Week_6 31 /* Book record: demonstrating linked list */ #include <stdio.h> #include <stdlib.h> struct Book { struct Book *next; // pointers to next struct in list int barcode; float price; int main() { struct Book *head = NULL; // head of list struct Book *prev, *current; int i=1; ICT106_Prac_Week_6 32 do { /* create memory block for Book structure */ current = (struct Book *) malloc(sizeof(struct Book)); if(head==null) { head = current; // first structure else { prev->next = current; // subsequent structure printf("\nenter barcode for %d book: ", i); scanf("%d%*c", &current->barcode); printf("enter price for %d book: ", i); scanf("%f%*c", &current->price); current = head; // pointer to head of list while(current!=null) { printf("book: %d Price: %f\n", current->barcode, current->price); /* pointer to subsequent struct */ current = current->next; current->next = NULL; // set the next struct to NULL prev = current; // update prev pointer i++; while (i<5); return 0; ICT106_Prac_Week_6 33 ICT106_Prac_Week_6 34 6